FIG. 10 provides a block diagram illustrating an exemplary configuration of a general optical disk recording/reproducing apparatus. As shown in FIG. 10, light emitted from a laser, which serves as a light source installed on laser diode unit (hereinafter, referred to as “LDU”) 102, is converted into collimated light by collimating lens 103 and then passes through polarization hologram 104. The linearly polarized light from polarization hologram 104 is then converted into circularly polarized light by quarter-wave plate 105 and focused on optical disk 101 by objective lens 106 driven by actuator 107, optical disk 101 being rotated by spindle motor 108. Objective lens 106 can be moved by actuator 107 in focusing direction F and tracking direction (track crossing direction) T. The light reflected by optical disk 101 passes through objective lens 106 and then is converted by quarter-wave plate 105 from the circularly polarized light to linearly polarized light, the polarization direction thereof being perpendicular to that of the linearly polarized light impinged upon quarter-wave plate 105 from polarization hologram 104. Thereafter, the light is diffracted by polarization hologram 104 and then passes through collimating lens 103. Next, the light is received by a photodetector serving as a light-receiving element on LDU 102. The photodetector outputs to pre-amplifier 121 a detection signal of the incident light thereon. Pre-amplifier 121 generates, from the detection signal, a focus error signal (hereinafter, referred to as “FE signal”), a tracking error signal (hereinafter, referred to as “TE signal”) and an RF signal, as will be described in detail later.
The FE signal indicates that a beam spot formed on a data recording layer of optical disk 101 is not in a specific focus state due to the deviation of objective lens 106 from a proper focus position in focusing direction F. The TE signal indicates that the beam spot is shifted in tracking direction T due to the deviation of objective lens 106 from a proper tracking position in the tracking direction T. The RF signal has data information recorded as pits or marks in the data recording layer of optical disk 101 and address information of tracks to or from which data is recorded or reproduced. Signal processing unit 122 receives the RF signal, and then extracts and reproduces the recorded data information and the address information. Servo unit 123 receives the FE signal and the TE signal and then generates a control signal for the control of actuator 107 to control objective lens 106 based on the control signal. Servo unit 123 also controls spindle motor 108. Laser driving unit 125 controls an output power of the laser on LDU 102 to be used in recording or reproducing data. The information reproduced by signal processing unit 122 is transmitted to controller 124. Servo unit 123 and laser driving unit 125 are operated under the control of controller 124.
FIG. 8 depicts an FE signal detection unit using a general spot size detection (hereinafter, referred to as “SSD”) method. Photodetector 201 is provided on LDU 102 and detects the FE signal. The FE signal is generated based on the output from photodetector 201. Referring to FIG. 8, the FE signal of “(b+c)−(a+d)” is generated by subtractor 204, and a summation signal, i.e., an FS signal of “a+b+c+d”, is generated by adder 205. From the FS signal, the RF signal is obtained. Reference numeral 202 represents a spot where detected light from the optical disk is focused in front of a detection surface of photodetector 201; and reference numeral 203 represents a spot where detected light from the optical disk is focused behind a detection surface of photodetector 201. When the distance between the optical disk and the objective lens changes, the size of one of spots 202 and spot 203 increases and the size of the other spot decreases. In this manner, the FE signal is generated.
When a disk having a groove, such as a DVD±R, a DVD-RAM or the like, is reproduced, zeroth-order light and first-order light are generated due to a diffraction by the groove. A guide groove of the above-described optical disk has protrusions (lands) and recesses (grooves). FIGS. 3A and 3B provide schematic diagrams showing distributions of detected light from an optical disk on a photodetector. Reference numerals 301 and 302 indicate light distributions in case of reproduction from a land and a groove reproduction, respectively. As shown in FIGS. 3A and 3B, the light distributions are inverted in case of the land and the groove reproduction. Instead of the full circle represented by reference numerals 301 or 302, a quarter sector shaped portions 902 and 903 focused on photodetector 901 as shown in FIG. 9 can be used as light spots for focus control, for example. Further, depending on the positional relationship between the photodetector and the spot, a variation in an amplitude of the FE signal due to defocusing may be changed for the land and the groove.
FIGS. 4A and 4B depict FE signals in case of changing the distance between the optical disk and the objective lens at the centers of a land track and a groove track, respectively. Dashed lines in FIGS. 4A and 4B represent FE signals at in-focus points.
FIG. 5 is a graph describing open-loop characteristics of a focus servo at a specific servo band frequency in case of changing a defocusing amount while setting an identical circuit gain for the land and the groove. When the FE signal has an inflection point near the in-focus point as shown in FIGS. 4A and 4B, focus gains in the land and the groove vary as shown in FIG. 5. Therefore, when variations in jitters and RF amplitudes with respect to a variation in the FE signal are measured, sensitivities in the land and the groove are different from each other, as shown in FIGS. 6A and 6B. In FIGS. 6A and 6B, curves opening upwards and downwards indicate jitter characteristics and RF amplitude characteristics, respectively.
If the focus control is performed while setting an identical circuit gain for the land and the groove when there exists an optical gain difference of the focus servo between the land and the groove, the focus servo may oscillate in the land due to its high gain or a focus control error may increase in the groove due to its low gain, depending on the setting. Therefore, it is preferable to set different circuit gains for the land and the groove.
Japanese Patent Laid-open Applications Nos. H7-129975 and H8-329484 disclose therein a gain-switching unit of a focusing error signal and a tracking error signal switching between the land and the groove.
Moreover, Japanese Patent Laid-open Application No. H10-91976 proposes a technique for setting gains in a land and a groove based on sensitivities calculated by using a focusing error signal and a tracking error signal obtained while driving an objective lens.
However, in order to measure open-loop gains of a servo system, it is required to use an expensive measuring equipment, e.g., a frequency characteristic analyzer or the like, or a circuit having a function same as that of the frequency characteristic analyzer, thereby increasing an installation space, a size of a circuit and costs.